SI-6123: -explaintypes should not explain errors which won't be reported

-explainTypes means that only type tests which *fail* should be reported in more
detail by using explainTypes. Hence, callers of explainTypes should check if
type errors are being ignored, by checking context.reportErrors. Hence, this
check is added to Inferencer, and another call site is redirected to that
method.

Moreover, explainTypes should only be called if an error exists. Enforce that in
checkSubType, and remove spurious home-made explainTypes output.

Finally, in ContextErrors, stop checking `settings.explaintypes.value` before
calling `explainTypes` which will check it again.

Note that this patch does not fix all occurrences, but only the ones which
showed up during debugging. The other ones never cause problems, maybe because
they occur when contextErrors is in fact guaranteed to be true. We might want to
fix those ones anyway.

This fixes regressions in c800d1f and
78f9ef3.

Thanks to hubertp (Hubert Plociniczak) for the first round of review.

Refs #6123
backport to _2.10.x_

src/compiler/scala/tools/nsc/typechecker/ContextErrors.scala

@@ -173,8 +173,7 @@ trait ContextErrors {
         assert(!foundType.isErroneous && !req.isErroneous, (foundType, req))
 
         issueNormalTypeError(tree, withAddendum(tree.pos)(typeErrorMsg(foundType, req, infer.isPossiblyMissingArgs(foundType, req))) )
-        if (settings.explaintypes.value)
-          explainTypes(foundType, req)
+        infer.explainTypes(foundType, req)
       }
 
       def WithFilterError(tree: Tree, ex: AbsTypeError) = {
@@ -1274,11 +1273,12 @@ trait ContextErrors {
     // not exactly an error generator, but very related
     // and I dearly wanted to push it away from Macros.scala
     private def checkSubType(slot: String, rtpe: Type, atpe: Type) = {
-      val ok = if (macroDebugVerbose || settings.explaintypes.value) {
-        if (rtpe eq atpe) println(rtpe + " <: " + atpe + "?" + EOL + "true")
+      val ok = if (macroDebugVerbose) {
         withTypesExplained(rtpe <:< atpe)
       } else rtpe <:< atpe
       if (!ok) {
+        if (!macroDebugVerbose)
+          explainTypes(rtpe, atpe)
         compatibilityError("type mismatch for %s: %s does not conform to %s".format(slot, abbreviateCoreAliases(rtpe.toString), abbreviateCoreAliases(atpe.toString)))
       }
     }

src/compiler/scala/tools/nsc/typechecker/Infer.scala

@@ -313,8 +313,10 @@ trait Infer extends Checkable {
       */
     )
 
-    def explainTypes(tp1: Type, tp2: Type) =
-      withDisambiguation(List(), tp1, tp2)(global.explainTypes(tp1, tp2))
+    def explainTypes(tp1: Type, tp2: Type) = {
+      if (context.reportErrors)
+        withDisambiguation(List(), tp1, tp2)(global.explainTypes(tp1, tp2))
+    }
 
     /* -- Tests & Checks---------------------------------------------------- */